1. Field of the Invention
The present invention relates to an auto focusing device used in a publicly used compact photographing device.
2. Description of the Related Art
A triangular method for measuring distance or an infrared distance measurement method has been adopted for auto focusing in a compact photographing device such as a camera. Recently, various products have been developed in which a video signal output from image sensor means is directly used for focusing. This is called a mount climbing-like method for measuring distance. The principle of this method is that the high frequency component of the video signal output from the image sensor is maximized when the camera becomes in focus.
An example of the distance measurement method is described below.
A subject is photographed by an image pickup system which outputs a video signal. The signal is amplified by a preamplifier.
The video signal from the preamplifier is changed to three signals, i.e., a red signal R, a green signal G, and a blue signal B by a signal processing circuit. A display device displays the image of the subject in accordance with the R, G and B signals. A band pass filter (BPF) filters the video signal from the preamplifier to pick up the high frequency component from the signal. A calculation unit calculates an in focus information data which represents the focusing state of the image pickup system on the basis of the high frequency component signal output through the filter. After that, a motor control unit drives a motor in response to the in focus information data transmitted from the calculation unit to move the lenses of the image pickup system so that the system becomes in focus.
In accordance with the above-mentioned auto focusing device, depending on the subject, a false peak arises wherein the high frequency component of the video signal from the image pickup system is temporarily maximized at one timing though the system is not in the in focus state. When the false peak arises, the calculation unit outputs an in focus information signal so that the image pickup system is focused on the false peak.
To avoid this problem, for example, an in focus discrimination method is proposed wherein the calculation unit discriminates the in focus state in such a way that a plurality of band pass filter are used to simultaneously filters a plurality of frequency components of the video signal output from the image pickup system and the system is regarded as being in focus when all of the components are at their peak.
As mentioned above, in accordance with the auto focusing device mentioned above, the calculation unit outputs the in focus signal in response to the false peak, which results in that the image pickup system is wrongly focussed to the false peak.
Also, in accordance with the in focus discrimination method mentioned above, the problem is not sufficiently solved and other problems arise. That is, the number of the parts is increased since a plurality of BPFs are used. Also, each frequency component of the video signal from the image pickup system is detected by a fixed band pass filter which results in that the characteristic of the subject image is limited. That is, the desirable way of obtaining the in focus information signal can not be conducted, which way is to detect as many frequency components as possible and select a component according to the image of the subject since the characteristics of the video signal from the image pickup system, such as height of the peak and the sharpness thereof, are different according to the subject. If the desirable way is to be achieved, the number of the parts must be further increased as well as the cost of the device.